A term that lazy parents use to describe people who "discriminate" against their rude and loud children. Some parents believe that their children should be allowed to do whatever they want, because kids will be kids. This philosophy requires the belief that children can do no wrong because they don't know better, the parent isn't responsible for teaching them any better, and everyone else has to put up with it.

Any adult who protests because a strange child screamed through their entire dinner in a restaurant or cried through an entire R-rated movie at a theater is a child hating adultist.
OR
An elderly lady is in line at the post office. A ten year old hits her repeatedly in the leg with a nerf bat. The parent does nothing. The elderly woman asks the child to please stop. The parent screams at the lady for being rude to her child and accuses her of adultism.
OR
A child runs behind the counter at a coffee shop, trips the employees, and knocks over hot drinks and glass cups. The shop owner asks the parent to keep her child from behind the counter. The mother accuses the establishment of discrimination against her child (adultism).

Noun. A disease that causes otherwise fun people to start acting like adults. This means they no longer go out and drink until they black out, stop having casual sex, and stop buying shoes and purses. There is no cure for Adultism.

Sally used to be so much fun, what happened?
She caught Adultism.
OMG, no wonder she's so boring!